Next generation transmission systems are anticipating bit rates approaching 100 Gb/s in time division multiplexed architectures. The demand for such speeds is created by the growth of interactive multi-media services and is made possible by the terahertz bandwidth of optical fiber. Realizing optical sources with these modulation bandwidths remains a significant obstacle however. The state-of-the-art edge-emitting semiconductor lasers have 3 db bandwidths in the region of 30 GHz. The limitations on this bandwidth arise from the non-linear gain mechanism and from the maximum values of differential gain that can be realized. The non-linear gain effect is due to the presence of current transport in the SCH(separate confinement heterostructure) regions and the associated dynamic response. The SCH regions perform the functions of both carrier confinement and optical confinement. For the purposes of carrier confinement, these regions can be reduced substantially. However, in the edge emitter these regions cannot be reduced to much less than about 1500.ANG. in thickness and yet still maintain a reasonable value for .GAMMA., the optical confinement factor for the quantum well in the optical waveguide. Also, as the low index waveguide regions are placed closer to the quantum well, the large index difference interfaces produce larger waveguide loss.
The differential gain is established by the differential stimulated lifetime which is essentially equal to (or slightly less) than the photon lifetime given by ##EQU1## where the parameters are R, the reflectivity of the cleaved facets and L, the length of the laser. The laser bandwidth varies inversely with .tau..sub.p. To increase speed either L or R must be reduced. With the semiconductor to air reflectivity fixed at about 0.3, the laser length cannot be reduced below about 200 .mu.m (by cleaving or by using dry etched mirrors). This limit is imposed by the rapidly rising threshold current and the reduced power capability. The output power is limited by the total device volume which is being reduced with the length. This tradeoff between speed and power with reduced length (constant power.times.bandwidth product) is common to all devices.
The vertical cavity laser has emerged as an alternative to the edge emitting laser because it allows a different mode of operation. In this case the mirror reflectivity is increased very close to unity and the cavity length is reduced to 1-3 wavelengths. Therefore the mirror loss term in the photon lifetime, i.e. ln(1/R)/L may be designed to be essentially the same as in the edge emitter. The term .alpha. representing optical loss is no longer limited by waveguide loss as in the edge emitter but rather by scattering in the mirrors and at the edges of the vertical cavity. Therefore the widths of the SCH layers in the vertical cavity growth are no longer important for determining .alpha. and can be reduced for the purposes of reducing the non-linear gain effect and therefore of increasing the bandwidth. Also, reducing the thicknesses of the SCH layers does not compromise .GAMMA. the optical confinement factor as it does in the edge emitter, since .GAMMA. is now determined only by the volume ratio of active layer to cavity size and position of active layer with respect to the standing wave and not by the waveguide parameters. Therefore the vertical cavity device can be optimized to reduce the "non-linear gain" effect and the mirror can be flexibly designed to trade-off the number of reflector pairs for a lower .tau..sub.p. Lower .tau..sub.p implies lower differential stimulated lifetime (.tau..sub.st *) and therefore larger bandwidth due to the increase in differential gain at the expense of a larger threshold current. The larger threshold, of course, results in lower maximum power due to the reduced current range for optical output and the increase in internal device heating.
In spite of the potential for high speed offered by the vertical cavity devices, the maximum reported bandwidths have been about 15 GHz. The limiting factor in these advanced structures has been the RC time constant of the device. In the edge-emitting laser, this bandwidth limitation results from the device parasitic capacitance (bond pad plus intrinsic PIN capacitance) and the output resistance of the measurement system because the device series resistance can be made very small. In the vertical cavity device, the device series resistance cannot be made negligibly small because the conduction is either forced through part of a DBR mirror or suffers from current crowding effects. Current crowding results because the current must flow two dimensionally from an adjacent region into the optically active area. The two dimensional flow is required since the current flow and optical emission are necessarily along the same axis.
Vertical cavity devices also have an output which is randomly polarized. It would be very useful to have a means to predetermine and maintain the polarization. Another impediment to vertical cavity deployment is the coupling of the light to fibers. Current methods use polymer waveguides or mirrors to redirect the light from the vertical to the horizontal direction. This is not a cost effective approach.
Another requirement of a very high speed laser is integration with driving electronics. At speeds of 100 GHz, hybrid connections of lasers and transistors become costly and impractical. A cost effective solution requires integrated devices. Therefore solutions to achieve 100 GHz laser operation to be successful practically will be part of an integrated optoelectronic technology.
The limitations of the laser in terms of matching impedances between the optoelectronic device and the electronic interface are the same for a laser or for a detector. Therefore the solutions to eliminating undesirable reflections for the laser may also be used for a detector
It is an object of this invention to provide vertical cavity operation of a laser which is implemented with the geometry of an edge emitter and so is capable of output powers proportional to the total volume of the laser cavity which can be very large, without sacrificing bandwidth.
It is an object of this invention to provide a vertical cavity laser in which the speed is not limited by the input impedance of the laser. Therefore, the goal of the invention is to introduce a traveling wave concept for both a laser and a detector which can eliminate the reflections produced at mismatched interfaces.
It is an object of this invention to provide a very high brightness source by concentrating all the power from a large volume optical cavity through a small output cross-sectional area.
It is an object of this invention to produce a predictable and stable polarization for the output of a vertical cavity laser.
It is an object of this invention to eliminate the effect of back reflections from fiber or waveguide interfaces upon the stability of the laser output. This is the function of an optical isolator.
It is an object of this invention to optimize and facilitate the coupling from a vertical cavity laser to a fiber by converting the vertical cavity power to waveguide power where the waveguide construction and parameters provide for a large misalignment tolerance.
It is a final object of this invention to provide the edge emitting output of a vertical cavity laser in the format of a optoelectronic integrated circuit so that the functionality of electronic and optoelectronic devices is merged.